A thermoelectric phenomenon, which is a reversible and direct energy conversion phenomenon between heat and electricity through a solid-state material, is a phenomenon generated by movement of electrons or holes in a thermoelectric material. The thermoelectric phenomenon can be explained with a Peltier effect in which heat is emitted or absorbed when applying current from the outside, a Seebeck effect in which electromotive force is generated from a difference in temperature between both ends of a material, and a Thomson effect in which heat is emitted or absorbed when a current flows in a material having a predetermined temperature gradient.
When using the Peltier effect, a cooling system which does not require a gas compressor and refrigerant gas may be implemented. Further, when using the Seebeck effect, heat generated in a computer, a vehicle engine, or the like, or waste heat generated in various industries may be converted into electric energy.
Recently, as a necessity for a technology of improving energy usage efficiency including vehicle fuel efficiency has increased, an interest in a power generation system using the thermoelectric material has also increased. For example, thermoelectric cooling and thermoelectric power generation efficiency may be directly connected with performance of the thermoelectric material, and to overcome a current limitation of the thermoelectric material restrictively used in a small and special cooling field, development of a high performance material is demanded.
In the related art, thermoelectric energy conversion efficiency indicating performance of the thermoelectric material is presented by dimensionless figure of merit (ZT) represented by the following Equation 1.
                    ZT        =                                            S              2                        ⁢            σ            ⁢                                                  ⁢            T                    κ                                    Equation        ⁢                                  ⁢        1            
In Equation 1, S is a Seebeck coefficient, σ is an electrical conductivity, T is an absolute temperature, and κ is a thermal conductivity.
To increase thermoelectric efficiency (i.e. ZT), a thermoelectric material simultaneously having a high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity may be required. However, since the Seebeck coefficient and the electrical conductivity have a trade-off relationship with a carrier concentration and the electrical conductivity and the thermal conductivity are not independent variables but are affected by each other, it may be complicated to implement a material having high ZT.
One of important strategies for improving performance of the thermoelectric material which is to manufacture a nanocomposite, may be manufacturing a nanograin structure of which a density of a grain boundary is increased by decreasing a grain size to a nano size or manufacturing a nanocomposite in which a phase boundary between a thermoelectric matrix and a secondary phase is formed by introducing the secondary phase having a nano size. In particular, thermal conductivity may be decreased by increasing phonon scattering in the grain boundary and the phase boundary, and the trade-off relationship between the Seebeck coefficient and the electrical conductivity is broken by a carrier filtering effect, thereby making it possible to improve ZT.
A nanostructure may be manufactured in a form of a zero-dimensional quantum dot, a one-dimensional nanowire, a two-dimensional nano plate, and a superlattice thin film, but a nanostructure material providing high ZT in a bulk form is required for actual application.
The above information disclosed in this Background section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.